(1) Field of the invention
The present invention relates to a method and apparatus for detecting a phase difference between two digital signals, such as a horizontal synchronizing signal and a system cock which has a three or four times higher frequency than a chrominance subcarrier in a television set. Such phase difference data is necessary for a digital phase detection circuit, especially a digital phase locked loop (PLL) circuit which synchronizes or locks the external signal,
(2) Description of the Related Art
Generally, in a conventional circuit for detecting a phase difference between two analog signals, in other words in a conventional phase comparator circuit for analog signals, it is necessary to add an external capacitor or resistor outside the phase comparator circuit unit. Thus, it is very difficult to integrate the conventional phase comparator circuit for analog signals into a small IC (Integrated Circuit) package. Further, the use of analog elements such as a capacitor C or resistor R in the phase comparator circuit cannot guarantee accuracy of the signal as compared with the accuracy in a digital circuit. Furthermore, the analog circuit is highly susceptible to interference caused by noise. For those reasons, the conventional phase comparator circuit for analog signals is not suitable as a phase comparator circuit for digital signals.
Nevertheless, in a digital PLL circuit, such as a circuit for processing a horizontal synchronizing signal of a digital television set, phase difference information among digital signals is frequently necessary for operation, so it is required to provide a small size phase comparator circuit for detecting a phase difference between two digital signals with guaranteed accuracy.